[PATCH 7/9] mm: Free up PG_slab

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

Reclaim the Slab page flag by using a spare bit in PageType.  We are
perennially short of page flags for various purposes, and now that
the original SLAB allocator has been retired, SLUB does not use the
mapcount/page_type field.  This lets us remove a number of special cases
for ignoring mapcount on Slab pages.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 include/linux/page-flags.h     | 21 +++++++++++++++++----
 include/trace/events/mmflags.h |  2 +-
 mm/memory-failure.c            |  9 ---------
 mm/slab.h                      |  2 +-
 4 files changed, 19 insertions(+), 15 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 94eb8a11a321..73e0b17c7728 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -109,7 +109,6 @@ enum pageflags {
 	PG_active,
 	PG_workingset,
 	PG_error,
-	PG_slab,
 	PG_owner_priv_1,	/* Owner use. If pagecache, fs may use*/
 	PG_arch_1,
 	PG_reserved,
@@ -524,7 +523,6 @@ PAGEFLAG(Active, active, PF_HEAD) __CLEARPAGEFLAG(Active, active, PF_HEAD)
 	TESTCLEARFLAG(Active, active, PF_HEAD)
 PAGEFLAG(Workingset, workingset, PF_HEAD)
 	TESTCLEARFLAG(Workingset, workingset, PF_HEAD)
-__PAGEFLAG(Slab, slab, PF_NO_TAIL)
 PAGEFLAG(Checked, checked, PF_NO_COMPOUND)	   /* Used by some filesystems */
 
 /* Xen */
@@ -931,7 +929,7 @@ PAGEFLAG_FALSE(HasHWPoisoned, has_hwpoisoned)
 #endif
 
 /*
- * For pages that are never mapped to userspace (and aren't PageSlab),
+ * For pages that are never mapped to userspace,
  * page_type may be used.  Because it is initialised to -1, we invert the
  * sense of the bit, so __SetPageFoo *clears* the bit used for PageFoo, and
  * __ClearPageFoo *sets* the bit used for PageFoo.  We reserve a few high and
@@ -947,6 +945,7 @@ PAGEFLAG_FALSE(HasHWPoisoned, has_hwpoisoned)
 #define PG_table	0x00000200
 #define PG_guard	0x00000400
 #define PG_hugetlb	0x00000800
+#define PG_slab		0x00001000
 
 #define PageType(page, flag)						\
 	((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
@@ -1041,6 +1040,20 @@ PAGE_TYPE_OPS(Table, table, pgtable)
  */
 PAGE_TYPE_OPS(Guard, guard, guard)
 
+FOLIO_TYPE_OPS(slab, slab)
+
+/**
+ * PageSlab - Determine if the page belongs to the slab allocator
+ * @page: The page to test.
+ *
+ * Context: Any context.
+ * Return: True for slab pages, false for any other kind of page.
+ */
+static inline bool PageSlab(const struct page *page)
+{
+	return folio_test_slab(page_folio(page));
+}
+
 #ifdef CONFIG_HUGETLB_PAGE
 FOLIO_TYPE_OPS(hugetlb, hugetlb)
 #else
@@ -1121,7 +1134,7 @@ static __always_inline void __ClearPageAnonExclusive(struct page *page)
 	(1UL << PG_lru		| 1UL << PG_locked	|	\
 	 1UL << PG_private	| 1UL << PG_private_2	|	\
 	 1UL << PG_writeback	| 1UL << PG_reserved	|	\
-	 1UL << PG_slab		| 1UL << PG_active 	|	\
+	 1UL << PG_active 	|				\
 	 1UL << PG_unevictable	| __PG_MLOCKED | LRU_GEN_MASK)
 
 /*
diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
index d55e53ac91bd..e46d6e82765e 100644
--- a/include/trace/events/mmflags.h
+++ b/include/trace/events/mmflags.h
@@ -107,7 +107,6 @@
 	DEF_PAGEFLAG_NAME(lru),						\
 	DEF_PAGEFLAG_NAME(active),					\
 	DEF_PAGEFLAG_NAME(workingset),					\
-	DEF_PAGEFLAG_NAME(slab),					\
 	DEF_PAGEFLAG_NAME(owner_priv_1),				\
 	DEF_PAGEFLAG_NAME(arch_1),					\
 	DEF_PAGEFLAG_NAME(reserved),					\
@@ -135,6 +134,7 @@ IF_HAVE_PG_ARCH_X(arch_3)
 #define DEF_PAGETYPE_NAME(_name) { PG_##_name, __stringify(_name) }
 
 #define __def_pagetype_names						\
+	DEF_PAGETYPE_NAME(slab),					\
 	DEF_PAGETYPE_NAME(hugetlb),					\
 	DEF_PAGETYPE_NAME(offline),					\
 	DEF_PAGETYPE_NAME(guard),					\
diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 9349948f1abf..1cb41ba7870c 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1239,7 +1239,6 @@ static int me_huge_page(struct page_state *ps, struct page *p)
 #define mlock		(1UL << PG_mlocked)
 #define lru		(1UL << PG_lru)
 #define head		(1UL << PG_head)
-#define slab		(1UL << PG_slab)
 #define reserved	(1UL << PG_reserved)
 
 static struct page_state error_states[] = {
@@ -1249,13 +1248,6 @@ static struct page_state error_states[] = {
 	 * PG_buddy pages only make a small fraction of all free pages.
 	 */
 
-	/*
-	 * Could in theory check if slab page is free or if we can drop
-	 * currently unused objects without touching them. But just
-	 * treat it as standard kernel for now.
-	 */
-	{ slab,		slab,		MF_MSG_SLAB,	me_kernel },
-
 	{ head,		head,		MF_MSG_HUGE,		me_huge_page },
 
 	{ sc|dirty,	sc|dirty,	MF_MSG_DIRTY_SWAPCACHE,	me_swapcache_dirty },
@@ -1282,7 +1274,6 @@ static struct page_state error_states[] = {
 #undef mlock
 #undef lru
 #undef head
-#undef slab
 #undef reserved
 
 static void update_per_node_mf_stats(unsigned long pfn,
diff --git a/mm/slab.h b/mm/slab.h
index d2bc9b191222..457b15da2a6b 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -84,8 +84,8 @@ struct slab {
 		};
 		struct rcu_head rcu_head;
 	};
-	unsigned int __unused;
 
+	unsigned int __page_type;
 	atomic_t __page_refcount;
 #ifdef CONFIG_MEMCG
 	unsigned long memcg_data;
-- 
2.43.0